Frequency-control system



Jan. 13, 1931. w. A. MARRISON 1,788,533

FREQUENCY CONTROL SYS TEM Filed March 28, 1927 3 Sheets-Sheet, 1

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FREQUENCY CONTROL SYSTEM Filed March 28, 19? s Smets-sheetY 2 Q 0 77MEHaz ,47m/wey Jan. 13, 1931. W. A.

FREQUENCY Filed March 28, 1927 MARRISON 1,788,533

CONTROL SYSTEM 3 Sheets-Sheet 5 Patented Jan. 13, 1931 UNITED STATESPATENT OFFICE WARREN A. uannrson; or omen, Naw JERSEY, AssIGNonf 'roBELL TELEPHONE LABORATORIES. INCORPORATED. 0F NEW YORK. N. Y., ACORPORATION 0F NEW YORK FREQUENCY-CONTRI; SYSTEM Application led March28, 1927. Serial 30.178,98?.

This invention relates to regulation, and especially to the control ofelectrical characteristics, as for example the frequency of electricalvariations.

A specific form of the invention, which is shown in the accompanyingdrawing and which has been chosen merely for the purpose ofillustration, is a system for maintaining the frequency of an electricspace discharge oscillator constant at a value which manner by means ofa piezo-electric crystal of readily obtainable size.

In this form of the invention a master oscillator has its' frequencymaintained constant in the usual manner by a crystal, but at a multipleof the frequency to be ultimately obtained, and the constancy of themaster oscillator frequency is utilized to maintain another oscillatorat the desired constant frequency, which is a submultiple of the masteroscillator frequency. To obtain the constancy of the frequency of thesubmultiple frequency oscillator, voltages derived from the masteroscillator and the submultiple frequency oscillator are so combined inan electric space discharge modulator as to produce current themagnitude of which varies with their instantaneous phase relation. rlhecurrent is caused to so controlV reactance in the tuned oscillationcircuit of the submultiple frequency oscillator as to oppose anytendency of that oscillator to vary its frequency with respect to thefrequency of the master oscillator and prevent the phase relationbetween the combining voltages from changing except by a small amountdepending on the degree of control required. The maximum phase variationobserved is about 180 at the frequency of the master oscillator.

llt is not necessary that the controlled frequency be an integralsubmultiple or integral multiple of the controlling frequency. In fact,each of the embodiments of the invention specifically shown anddescribed herein by way of example is a system for maintaining thefrequency of an oscillator at any frequency which is a rational fractionof a given controlling frequency. The control is greatest when thefraction is Z/n or n/Z where n is a small integer, but is appreciablewhen the fraction is n/m, n and m oth being integers.

In one form of the invention the controlled oscillator itself generatesa voltage which is employed as one of the two ycombining voltages, andthe master oscillator itself serves as the modulator for combining thevoltages to produce the current which controls the tuning reactance ofthe submultiple frequency oscillator. is too low to be maintained fixedin the usual Fig. 1 is a circuit diagram of one form of the invention;

`Fig 1A is a circuit diagram of a modified form;

Figs. 2, 8, 4 and 5 show urves for facilitat- Iing explanation of theintention; and

Fig. 6 is a circuit diagram of another form of the invention.

Referring to Fig. 1, HF indicates the master oscillator which is assumedto generate a wave of frequency which is high as compared with thecontrolled frequency and specifically an integral multiple thereofalthough there may be any rational fractional relationship Ibetween thetwo frequencies. lt may have its frequency maintained constant, as forexample by a quartz crystal Q,- incorporated in the oscillator in awellknown manner. An oscillator 0 of any standard type involving a tunedcircuit (L1, L2 and C2) may be used as the controlled, low frequency,source. An oscillator of a type disclosed 1n U. S. patent to erry,1,573,948, February 23, 1926, is shown, by' way of example, having inaddition only the controlling inductance L2.

Elements l, 2, 3 and 4 are electric space discharge tubes. rlube 1 is amodulator or rectifier tube giving a direct current output the amplitudeof which varies with the phase relation of the inputs. Tube 2 is the lowfrequency oscillator tube. rl`ube 3 supplies energy to output circuit 5at low frequency without reacting on the control circuit. Tube 4 is aharmonic producer. It is an amplifier with its operation so adjustedthat, in conjunction with transformers 10 and 11, it feeds a narrowpeaked wave to tube 1. The characteristics of the last tube should bechosen to suit any special case. When controllin a frequency about 1/5of the given hlgh frequency a Western Electric V tube (Code No. 102-D)with 9 volts C battery works very well. Preferably, some form of gaincontrol device, as for example the interstage voltage divider -9, shouldbe used to adjust the input to this tube, so that its space current willbe cut oif except during a sufficient portion of each cycle forproduction of the narrow, peaked Wave just mentioned. For example, spacecurrent may be permitted to 'low only during timescorresponding to thesmall fractions of the positive halfcycles of the impressed Wave in theneighborhood of the to s of these positive pulses. The function o such aharmonic producer may be conveniently conceived of as forming thesubharmonic Wave into an impulse Wave of the same frequency.

The direct current output of tube 1 flows through one winding 13 on themagnetic cored control coil 12. The core of the coil may be of iron orpermalloy, for example. Constants are so chosen that a small variationof this D. C. output causes the maximum change of inductance of theWinding L2 by virtue of the variable permeability of the core withmagnetization.

The frequency of the low frequency oscillator is adjusted to be thesubmultiple desired by means of condenser C2. It is then controlled byand kept in locked synchronism with the controlling input frequency. Ifsomething happens which tends to increase or decrease the frequency ofthe low frequency oscillator the change in D. C. produced thereby intube 1 changes the inductance of Winding L2 just enough to compensate,the only effect on'the output being a very slight phase shift withrespect to the control frequency.

-The condenser C1 prevents high frequency current from reaching thecontrol coil and therefore the low frequency ampliers. The resistance Rkeeps the impedance of the circuit of the direct current Winding of themagnetio cored control coil high so as to avoid any close approach toand inductively shortcircuiting of the other Winding by condenser C1 andplate battery 17, and thus obtain the largest possible variation ofinductance in Winding L2. The optimum condition may be determined Withthe help of meter 19. Condensers C3 and C4 are blocking condensers toprevent the voltage of the plate battery 17 from being connected toground through the oscillating coil and from reaching the grids of tubes3 and 4. The battery 18 supplies current for heating the filaments ofall of the tubes. Batteries 15 and 16 supply steady negative potentialsfor the grids of tubes 1 and 3, respectively.

For the Waves of the controlling and controlled oscillators of thesystem of Fig. 1, with their frequencies at, for. example, 25,000

cycles and 5000 cycles, respectively, the pattern observed on a Brauntube is stationary,

there being no hunting effects. The effect of varying the filamentcurrent or the voltage of battery 17 slightly is to cause a slight shiftin phase, which results in maintaining the frequency constant. Ifoperation of the system is stopped and started again, the control effectbegins a ain immediately.

Important a vanta es of this form of frequency control are t e greatstability obtained and the ease and stability of adjustment. Moreover, alarge output of the controlled frequency is obtained which ispractically free from harmonics. If harmonics are desired they may beadded in well known Ways, as for example, by an additional electricspace discharge tube modulator or harmonic producer. Only three tubes,1, 2 and 4, are required. The functions performed by the different tubesare related in such a Way that no interference will be introduced byoperating the filaments from alternating current.

Preferably the constants of the system are so adjusted as to cause thecontrol tube 1 to operate on the curved lower portion, shown in Figs. 2,3 and 4, of its grid voltage-plate current characteristic 20. Thesefigures are merely illustrative of the operation of the control tube forone condition of adjustment of the operating conditions of the system,chosen by Way of example. The steady grid voltage Ec of battery 15 isshown as just sufficient to reduce the space current of tube 1 to zerowhen there is no variable input to the tube. The graph 21A in Fig. 2represents instantaneous values of the plate current in the tube. Thisfigure represents conditions when the only variable voltage on the gridis the high frequency voltage Wave 21 in the secondary Winding oftransformer 11, due to the source HF. The graph 21A consists of onepulse of positive sign for each positive pulse of the voltage from HF.The graph 21B represents the average value of these pulses, which is thedirect current that they produce in the winding 13 of the control coil.The instant chosen as the zero or reference instant of time is indicatedby the zeros of the time scales.

Fig. 3 represents conditions when a voltage pulse 22 in phase oppositionto Wave 21 is received by the grid of tube 1 due to the Wave from tube4, it being assumed that the adjustments such as the setting of the gaincontrol device 9 and the value given to the voltage of battery 16 aresuch that the amplitude of the pulse 22 is equal to the amplitude ofwave 21 and the duration of the pulse is equal to one period of the Wave21 (although in reality the duration of the pulse usually wouldpreferably be considerably less) and it being assumed that the pulseoccurs during the second cycle of Wave 21 from the arbitrary zero orreference point of time. Further, for the sake of simplicity the shapeof the pulse is represented as though it .were

substantially the same as 'that of one cycle of wave 22, although inreality it usually would be somewhat narrower.- With the frequencies ofthe oscillators HF and O in the ratio 5:1 as considered above, the pulseoccurs every lifth cycle of the wave 21. Therefore, counting from thearbitrary zero or reference point or time, the pulses of graph 21A whichin Fig. 2 correspond in time to the 2nd, 7th, 12th, 17th, 22nd, etc.,positive half cycles of wave 21 will be absent Jfrom the graph 21A.,under the conditions represented in Fig. 3; for the effect ot' such haltcycles or wave 21 upon the grid of tube 1 is nullitied vein er rig. 2.

Fig. 4 represents conditions when the voltage pulse A22 occurs laterthan in Fig. 3 by one halt period of wave 21, so that the pulse is inphase with wave 21. Counting from the arbitrary zero or reference pointof time, the pulses of graph 21A corresponding in time to the 2nd, 7th,12th, 17th, 22nd, etc., positive half cycles of wave 21 will beaugmented in the graph 21A, under the 'conditions represented in Fig. 4;for the edect of such half cycles of wave 21 upon 'the grid of tube 1 isaugmented due to the fact that a positive half cycle of the pulse 22shifts the operating pointfor the half cycle of wave 22 upwardly tosteeper portions of the characteristic curve 20 and back again, orcauses 'the grid voltage due to wave 21 and pulses 22 to have aresultant value indicated by curve 23. Therefore, under the conditionsrepresented in Fig. 4 the direct current 21B in winding 13, which is theaverage value of all ot the current pulses in that winding, is greaterthan the current 21B in Fig. 2 by an amount depending on the curvatureof the'characteristic curve 20 in the operating region. i

Thus, Figs. 3 and 4 illustrate how the direct current, or the averagevalue of all ott the current pulses, in winding 13, can vary with thephase relation between the high frequency wave 21 and the pulses 22 fedto the grid of tube l by the harmonic generator 4. (This average valueof current also depends upon the number of cycles of high frequency foreach pulse 22 fed from the harmonic generator 4.) Variation of the phaserelation and consequently of the direct current, occurs when thefrequency of oscillator O tends to depart from a given value withrelation to the frequency of the oscillator HF; and the variation of thedirect current issuch as to check this tendency. For example, the normalcondition of operation of the `system operating on a ortion of itsmagnetizng current-permeabihty curve of negative slope. Then ifsomething causes the oscillator O to tend to increase its frequency, thepulses 22 advance slightly in phase with respect to wave 21, the directcurrent in winding 13 consequently decreases, the permeability of thecore of coil 12 therefore increases, and the inductance of winding L2therefore increases to oppose the tendency of the frequency etoscillator 0 to increase, and the frequency ot' the voltage wavedelivered to the output circuit 5 remains constant a't 5000 cycles/sec.although this wave has advanced slightly in phase with respect to thewave of oscillator HF. The phase advance is less than a half-period ofthe oscillator HF.l T something had caused the oscillator 0 to tend todecreaseits frequency instead of to increase it, the correcting-actionof the system would have been opposite to Vthat just described. Thus,the submultiple oscillator is kept in synchronism with the masteroscillator and adjusts itself to such va phase relation to it that therectified current from the vcontrol tube 1 suffices to change theinductance of the magnetic cored coil enough to adjust the frequency ofthe submultiple oscillator.

While Figs. 2, 3 and 4 amply illustrate the principle ot operation ofthe invention, it is true, as has already been indicated, that a muchgreater sensitivity of response tends to result from the use of pulseswhich are of shorter duration than those assumed in Figs. 2, 3 and 4 andespecially from the use of pulses which are of shorter duration than thehalt. periods of the controlling waves. This condition is illustrated byFig. 5 which assumes conditions which are exactly like those illustratedby Figs. 3 and 4 except only as to the duration of pulses. Forsimplicity, and because the figure is intended merely to illustrate analternative type of pulse, the

wave forms of the resultant rectified current are not shown'. They maybe plotted in exactly the manner disclosed in Figs. 3 and 4. It isevident that a control system of this type is more responsive than thesimpler type illustrated by Figs. 3 and 4. For instance, the relativephase of the pulse need change only about 90from the particular phasecondition illustrated by the figure, which results in a maximum directcurrent in winding 13 as in the case of Fig. 4, to achieve a conditionof minimum current. This result may not be approximated by the 'circuitsto be controlled.

The principle of the invention can be used to control harmonicfrequencies instead of submultiple frequencies, by reversing the controlprocess-that is, by having the oscillator which includes the coil 12 inits tuned frequency determining circuit the high frequency oscillatorand having the oscillator which is connected in circuit as oscillator HFis connected in circuit in Fig. -1 the relatively low frequencyoscillator.

The controlling effect in a system such as that of Fig. 1 can beincreased if a band-pass filter or tuned circuit 30 to select and passthe high frequency component corresponding to the frequency of the wavefrom source HF from the wave fed from tube 4, and preferably also an`amplifier 35, be introduced in the low frequency circuit feeding tube 1,as indicated by way of example in Fig. 1A. In that figure transformers112 and 113 replace the transformer 11 of Fig. 1. The portion of thesystem of Fig. 1A to the right of line X-X is the same as in the case ofFig. -1. Where, as in the case considered above, the ratio of thecontrolling and controlled frequencies is 5:1, the phase displace- -mentrequired between these frequencies to produce a given variation in thedirect current in winding 13 is approximately onefth as great in thecase of Fig. 1A as in the case of Fi 1. There results a greatereffectiveness o control.

In the circuit of Fig. l, one tube is used for each function. In somecases in the interest of simplicity it is preferable to operate withfewer tubes and makeeach tube performmore than one function.

The circuit of Fig. 6 will generate a submultiple of the naturalfrequency of the high frequency oscillator which, if desired, may haveits frequency set by a. piezo-electric crystal Q having the same naturalfrequency as the tuned circuit 40of the oscillator. The oscillatorcomprises an electric space discharge tube 1, operating on a non-linearpart of the grid yoltage-plate current characteristic. Because of thisthe oscillator functions also as a modulator or rectifier correspondingto the rectifier tube 1 of Fig. 1 and the space current will vary withthe phase relation of the wave of the crystal oscillator and a wave fedto the grid of the oscillator through coupling M1 as will now bedescribed.

The controlled submultiple-frequency or low frequency oscillator Ocomprises an electric space discharge tube 4 which also operates on, acurved portion of its characteristic in order to-impress harmonics ofthe low frequency on the grid of tube I1 through a circuit comprising anadjustable resistance 45 and coupling M1. Tube 4 thus performs functionsof both of the tubes 2 and 4 of Fig. 1, avoiding necessity for two tubesfor performing these functions. This tube 4 should have a large Cbattery 46, thereby making use of the lower part of the gridvoltage-plate current characteristic. A retard coil 47 is included inthe circuit which supplies direct space current from the plate battery17 to tube 4.

The operation of the system of Fig. 6 will be apparent without furtherdescription from the description, above, ofthe operation of the systemof Fig. 1.

Although only two oscillator-modulators are shown in Fig. 6, the systemmay be extended by adding other such oscillator-modulators, each havingits output circuit connected to the input circuit of the preceding oneas the output circuit of tube 4 is connected to the input circuit oftube 1, so that each oscillator succeeding the first in the series willhave its frequency controlled by the preceding oscillator as thefrequency of oscillator 4 is controlled by the frequency ofoscillator 1. Each oscillator of the series can thus have its frequencyan exact submultiple of the frequency of the preceding oscillator. Forexample, the oscillators may have frequencies of 100,000, 10,000 and1,000 cycles/sec., respectively, and the oscillators may have outputcircuits, respectively, each corresponding to circuit 5 but eachreceiving a wave of the frequency of the oscillator to which it isconnected.

One field of use of the invention is the measurement of frequencieswhich are too high to be measured directly and which it may therefore bedesirable to measure by a process involving one or more stages ofdownward frequency conversion. By employing this invention for theconversion, there is avoided any necessity for using entirelyindependent separate local sources, which introduce additional errors oruncertainties in the measurement.

What is claimed is:

1. In a method of frequency control, deriving from one of two wavesperiodic pulses individually of duration different from a half-period ofthe other wave, so combining the other wave with said pulses as toproduce current of magnitude varying in accordance with the phaserelation of the other wave to saidpulses, and controlling the frequencyof one of said waves in accordance with the magnitude of said current.

2. In a method of frequency control, deriving from one of two wavesperiodic pulses individually of duration 4gdierent from a half-period of the other wave so combining the other wave with said pulses as toproduce current of magnitude varying in accordance with the phaserelation of the other wave to lll' said pulses, and so controlling thefrequency of one of said waves in accordance with the magnitude of saidcurrent as to prevent phase change in said pulses measured by a timegreater than one half period of said other voltage. l

3.` ln a frequencycontrol system, sources of voltage of relatlvely highand relatively low frequencies respectively, means for deriving fromsaid relatively low frequency source a wave of narrow, peaked pulses ofvoltage each of duration less than a half cycle of the voltage of saidrelatively high frequency source, and means responsive to the relativelyhigh frequency voltage. and said pulses to control the frequency of saidrelatively low frequency source in accordance with the phase relation ofthe high frequency voltage to said pulses.

4. The method of controlling the ratio of the frequency of an electricenerator to the frequency of a given wave, t e frequency of -which isotherwise independent of that of said generator, which comprisesderiving from said generator an alternating current component whosefrequency is commensurable with the frequency of the given wave butdidering therefrom, so combining said Waves as to produce a directcurrent which is steady when the phase relation is constant and whichchanges when the phase relation changes, and exerting on said generatoran induence varying in accordance with said direct current and in such adirection as to prevent said generator from changing its frequency.

5. A system comprising a source of a wave of given frequency, anoscillator having a tuned frequency determining circuit'for setting thefrequency of said oscillator approximately at a rational fraction ofsaid first frequency, an electric space discharge. device responsive towaves from said oscillator for producing a narrow peaked wave of thefrequency of said oscillator, a second electric space discharge device,means rendering said latter device responsive to said narrow peaked waveand the wave from said source" to produce current of a magnitudedepending on the phase relation between said two latter waves, and meansresponsive to said Vcurrent for so controlling the natural period ofsaid tuned circuit as to maintain the ratio Aof said first frequency andthe frequency of said oscillator constant.

6. A frequency controlling system comprising a master oscillator, apiezo-electric crystal so connected to said oscillator as to maintainthe frequency of said oscillator constant, a second oscillator having atuned frequency determining circuit for setting the frequency of saidsecond oscillator approximately at a rational submultiple of thefrequency of said master oscillator, an electric space 4discharge deviceresponsive to WavesV from said second oscillator for producing a narrowpeaked wave of said submultiple frequency, a second electric spacedischarge device, means rendering said latter device responsive to saidnarrow peaked wave and the wave from said master oscillator to producecurrent 'of a magnitude depending on the phase relation between said twolatter waves,

Vand means responsive `to said current for so and which have a phaserelation var ing in' accordance with the tendency of t e frequency ofsaid submultiple oscillator to change its frequency with respect to thefrequency of said master oscillator, means for so combining saidvoltages as to translate said variations of phase relation into directcurrent the value of which varies with said phase relation and isappreciable when Athe waves of said voltages are so related that amaximum instantaneous value of one occurs simultaneously with a zeroinstantaneous value of the other, a frequenc determining inductance andcapacity inc uded insaid submultiple-frequency oscillator, said inductance having magnetic material in its magnetic circuit, and means forcausing said direct current to so vary the permeability of said materialas to maintain the frequency of said submultiple-frequcncy oscillatorexactly at said rational submultiple of the frequenc of said masteroscillator.

8. l frequency controlling system comprising a. master oscillator andmodulator, a second Yoscillator and modulator having its oscillationfrequency a rational fraction of the oscillation frequency of saidmaster os cillator, means for impressing a modulated wave from saidsecond oscillator upon said first oscillator, and means responsive to amodulation product from said'rst oscillator to control the frequency ofsaid second oscillator.

9. A frequency controlling system comprising an electric space dischargemaster oscillator, a piezo-electric crystal so connected i' outputcurrent' characteristic, meansicausi'ng llll said second oscillator tooperate on the lower curved portion of its input voltage-output currentcharacteristic, means for so impressing harmonics of said submultiplefre uency from said second oscillator upon sai oscillator as to cause.the space current of said first oscillator to vary with the phaserelation of the crystal oscillator wave and the wave impressed upon thecrystal oscillator from the second oscillator, and means responsive tosaid current for controlling the natural period of said tuned frequencydetermining circuit.

10. system comprising a source of variations of given frequency, anoscillator having a tuned frequency determining circuit for setting thefrequency of said oscillator approximately at a rational fraction ofsaid first frequency, an electric space discharge device responsive towaves from said oscillator for producing a narrow peaked wave of thefrequency of said oscillator, a second electric s ace discharge device,means rendering said latter device responsive to said narrow peaked waveand the wave of said source to produce current of a magnitude dependingon the phase relation between said two latter Waves, means responsive tosaid current for so controlling the natural period of said tuned circuitas to maintain the ratio of said first frequency and the frequency ofsaid oscillator constant, a load circuit, and a unidirectionallytransmitting device connecting said oscillator to said load circuit.

l1. A method of achieving a fixed frequency relation between waves fromtwo sources, which comprises deriving from the wave from one source aninterrupted wave whose periodicity of interruption is the frequency ofthe wave from which it is derived, combining said interrupted wave withthe wave from the other source so as to produce a direct currentcomponent of ma itude varying in accordance with the phase relation ofthe two combining waves, and controlling the frequency of the'wave fromone source in accordance with the magnitude of said current.

12. A frequency controlling system comprising an oscillator having afrequenc determining inductance, a second oscil ator, means responsiveto a relative change of the frequencies of the waves from saidoscillators for producing a direct current which varies in averageamplitude as in accordance with such relative change, and means usingsaid current for variably saturating said inductance and therebycontrolling the frequency of the wave from the rst oscillator so as tomaintain any denite predetermined' ra` tional relation between thefrequencies of the waves from said oscillators.

In witness whereof, I hereunto subscribe my name this 26th day of March,A. D. 1927.

WARREN A. MARRISON.

first Certificate of Correction Patent No. 1,788,533. January 13,19,31.`

` WARREN A'. MABRISON It is hereby certied that error ppers in theprinted lspeeiication of the yabove numbered patent requiringcorrectionas follows: Page 1, 1 1ne 50, for Z/n or n/Z read l/n or n/l;and that the said Letters Patent should be readwith this corroegiontherein that the same may conform to the record of the casein the PatentSigned and sealed vthis-14th day'oiMarch, A; D. 1933.

[wn] v f M. J. MOORE,

, Acting'om/mz'ssoner of Patents,

